When I submit a form, there is an error in the confirmation email. I have modified it in zp-core/locale/nl_NL/LC_MESSAGES/ zenphoto.po and zenphoto.mo

Now it displays the fields in the form in English and the confirmation email is also in English.

What's wrong?

I suspect you are referring to a typo or otherwise wrong string in the Dutch translation? That would be something for @fretzl to answer.

Perhaps you didn't modify the translations files correctly? Did you actually enable Dutch? Does your server have Dutch locale?

all the other text is in dutch just not the form fields and the comfirmation mail
Yes it was a typo: I ga akkoord met het opslaan en verwerken van mijn gegevens door deze website. instead of: Ik ga akkoord met het opslaan en verwerken van mijn gegevens door deze website.

The confirmation texts are actually option texts you can define manually. If you use only one language anyway I would suggest to modify those texts directly.

There is a workaround implemented with these editable texts so defaults should be translated but that is a bit buggy.

I've edited the text to correct the error.
In the files zenphoto.mo and zenphoto.po.
But since then he takes the English text instead of the Dutch for the form fields

Probably because the strings don't match exactly anymore. gettext can be quite specific with this. Probably everything needs to be update properly. Did you use Poedit for this? Sorry no other idea. @fretzl will surely update typo later on.

Thanks for finding the typo.
It's fixed in the Support build.